For applications in electric vehicles, a plurality of battery cells is usually combined with one another, wherein the battery cells are as a rule lithium ion (LION) cells. To this end, the battery cells are connected with each other in series, so that a battery stack of a plurality of cells with voltages of some hundred volts results. This battery stack is in turn divided into so-called battery blocks that may each be controlled and monitored by a separate control.
A so-called ASIC (application-specific integrated circuit) chip may, for instance, be used as a control. The object of these chips is to detect differences between the individual LION cells which are due to manufacturing and aging so as to be able to react appropriately, for instance, by specifically discharging some cells or blocks (passive balancing) or specifically redistributing charge between the cells or blocks (active balancing).
For an optimum control of the battery, as exact measuring values as possible about the voltages of the individual battery cells (cell voltages) have to be provided. Furthermore, any cell voltages have to be measured and monitored individually in such battery stacks with series connections alone for safety reasons. In doing so, a preferably simultaneous measuring of all cell voltages has to be aimed at.
In the corresponding battery blocks that comprise a number of 12 battery cells, for instance, different total voltages (common mode voltages) up to 60V may occur, which cause high demands to signal processing and measuring accuracy also for integrated circuits with high voltage technology.
The common mode voltage is defined as the average voltage of the positive and the negative voltage of a battery cell and hence corresponds to half the sum of the positive potential and the negative potential with respect to ground. The common mode voltage results, for instance, from the measured voltage of a battery cell with respect to ground, wherein a differential voltage is detected as a difference from the amount of the positive and the negative voltage. The common mode voltage interferes with cell voltages, so that one intends to eliminate it from the measurement procedure.
The common mode voltage is therefore no directly measurable voltage, but a mathematical voltage value that is used by the battery control. With the batteries in question, the common mode voltage may amount to up to 60 volts. Only the cell voltage can be measured, wherein the voltage of each battery may be different and each cell voltage may also have a different common mode level.
For measuring cell voltages in battery series connections, an analog digital converter (ADC) with a successive approximation register (SAR) or an ADC pursuant to the Sigma Delta (SD) method is used, wherein a respective HV multiplexer is used at the input thereof.
The construction of a measuring circuit with an analog digital converter and a successive approximation register has, however, as a consequence that a HV multiplexer is present in the signal path, which may have a disadvantageous effect on measuring accuracy. Moreover, the HV circuits of the multiplexer and the sample and hold members for the buffering of input voltages have a high space requirement on the integrated circuit.